This invention relates to magnetic tape drives, and more particularly to methods and apparatus for determining the presence of a file protect ring in a tape reel on the drive and also for determining the amount of tape that is contained on the tape reel. File protect rings are used to prohibit the erasing of information that was previously recorded on a tape. This is achieved by inserting the file protect ring in the reel that contains the tape that is to be protected. The tape drive on which that reel is subsequently mounted senses the presence of the file protect ring and in response thereto inhibits any write current in the write heads.
Other sensing apparatus are used in the tape drive to determine when the take-up reel is running out of tape. These sensors are included to enable a high speed rewind of the tape. During such a rewind operation, the tape is transported in a reverse direction at speeds which may be several times faster than the normal forward speed. Due to this high speed and the inertia of the tape reels, a relatively large time interval is required to stop the tape movement. This time interval is provided by sensing when the take-up reel is nearly empty and by initiating a tape stop sequence at that point.
In the prior art, separate sensors were provided for sensing the file protect ring and the amount of tape on the take-up reel. One of the sensors operated in conjunction with the supply reel for sensing the file protect ring; whereas the other sensor operated in conjunction with the take-up reel for sensing the amount of tape thereon. Typically, the take-up reel sensor included a light transmitter circuit and a light receiver circuit. The transmitter operated to emit a beam of light slightly above the take-up reel surface on which the tape was wound. For example, the beam of light might clear this surface by approximately 1/8 inch. Thus, when the amount of tape on the take-up reel was relatively large, the beam of light was blocked from passing over the surface; whereas when the amount of tape on the take-up reel was relatively small, the beam of light passed over the surface. The light receiver circuit was positioned to detect whether or not this beam of light was being blocked.
One problem with this prior art apparatus is that the alignment of the light beam with the take-up reel surface is highly critical. That is, if the light beam passes over the surface by too great a distance, then the rewind operation will slow down too soon; which in turn makes the operation of the drive slower. Conversely, if the light beam passes too closely to the take-up reel surface, then the rewind operation may result in the tape coming completely off of the take-up reel.
Still another problem with this prior art apparatus is that it is difficult to distinguish between when the light beam is actually being passed or blocked. As tape is unwound from the reel, the amount of light that passes increases in an analog fashion from no light to full light. And due to this continuous analog change in light intensity, the optimum point for initiating the end of the high speed rewind operation cannot be sensed.
Therefore, it is one object of the invention to provide a dual purpose sensor that operates in conjunction with only the supply reel for sensing both the file protect ring and the amount of tape on the take-up reel.
Another object of the invention is to provide a dual purpose file protect amount of tape sensor having improved accuracy.
Still another object of the invention is to provide a completely digital dual purpose file protect amount of tape sensor.